Physicists from the Massachusetts Institute of Technology (MIT) have successfully recreated one of the most important effects in quantum mechanics, known as the "Elitzur-Vaidman bomb tester." Remarkably, they accomplished this within a completely classical system, which means they observed a quantum effect in a system derived from everyday physics. This achievement intensifies the question: What truly separates the quantum from the classical?
In 1993, Israeli physicists Avshalom Elitzur from the Weizmann Institute of Science and Lev Vaidman from Tel Aviv University proposed the following thought experiment: Imagine you have a bomb with an ultra-sensitive trigger that detonates upon contact with a single particle. As long as the bomb is safely stored, no particle can touch the trigger, allowing us to control the timing of its detonation. However, bombs can malfunction, appearing like any ordinary bomb but failing to detonate when the trigger is touched.
Suppose we found an old bomb in storage and wanted to determine whether it functions properly. In everyday physics, the only way to test it is to attempt detonation: if functional, it will explode; if malfunctioning, it will remain intact. The problem with such a test is that it destroys functioning bombs, preventing their future use. We need a method that enables "both testing and not activating" the detonator.
Yes, No and Yes-No
The option "both yes and no" will sound familiar to anyone interested in quantum mechanics. Quantum particles can exist in a mixed state of multiple possible properties, such as not being near the bomb but also not too far away from it, remaining in a state that combines probabilities of being in any of these positions. This state is called a superposition. As long as the particle is not measured, meaning there is no external factor observing it and trying to determine its location, it will remain in this mixed state, between activating and not activating the bomb. Once we observe and measure its location, it will "collapse," ceasing to be in a mixed position and appearing in only one location, like any object in classical physics. This means that merely observing a particle affects its position. It should be noted that you cannot choose where the particle will collapse, but only cause the collapse.
Elitzur and Vaidman proposed a method to use a quantum particle to test the bomb without detonating it. They suggested taking a photon—a light particle—and passing it through a beam splitter. The photon's trajectory would be in a superposition between two possible directions, meaning the photon would be in a mixed state of two possible trajectories. One trajectory passes through the bomb, and the other does not touch the bomb. Next, the trajectories are diverted to meet in another beam splitter, oriented in reverse to the previous one, and the photon's trajectory is measured at the end.
Since the measurement is done at the end, by the time the particle reaches the second splitter, it is in a mixed state and does not "choose" a specific path. Only once the measurement is performed does the photon collapse into one specific path. It could collapse into the path with the bomb, causing an explosion, thereby indicating that we had a live bomb, which is now gone. However, it is also possible that it will collapse into the path without the bomb. In this case, the existence of the bomb will leave residual effects on the position of the photon, and we will be able to recognize them.
Technical explanation of the experiment
The bomb experiment demonstrates the use of basic quantum mechanics principles: superposition and measurement. Another important principle of quantum mechanics is wave-particle duality, which states that a quantum particle's mixed position also exhibits wave-like qualities. For this reason, passing a light particle through a beam splitter causes wave behavior, such as interference.
When a photon is passed through a beam splitter - an optical device - it behaves like a wave of light. If a single wave arrives, it will be split between two directions. If two waves arrive, they will be joined into one wave. This also applies to the photon—if it arrives in an absolute position with no superposition, its position will change and become mixed. If it arrives in a mixed position, it will change into an absolute position.
After the split, the photon is in superposition, meaning there is a fifty percent probability it will pass through the path where the bomb is located, and a fifty percent probability it will pass through the other path. Mirrors are added to direct the photon to another splitter. If the bomb is a dud, the photon split, encountered nothing of interest along its path, and was rejoined in the second splitter. Thus, the photon is not in a mixed position, as the second splitter turns the mixed position into an absolute position. Consequently, if the bomb is a dud, the photon will necessarily continue forward towards detector D.
If the bomb is live, there are two possibilities following the measurement. In the first scenario, with a fifty percent probability, the photon will pass through the bomb, causing it to explode, and the identification of the bomb will fail. This is the least interesting possibility. In the second scenario, with the same fifty percent probability, the photon will not pass through the bomb and will reach the second splitter, where it will be split. Now there is a fifty percent probability that we will be able to measure the photon in detector C. If we find the photon there, we will know with certainty that the bomb is live, even though it was not detonated by the photon.
Elitzur and Vaidman's method allows for the identification of live bombs in a quarter of the cases—compared to zero cases in classical physics. With further development, this success rate can be significantly improved.
Quantum experiment with classical methods
This idea is important, as it is one of the foundational examples of tasks achievable with quantum measures that are impossible with classical methods alone. It emphasizes the fundamental differences between classical mechanics and quantum mechanics: superposition and the effect of measurements on quantum systems. While the experiment itself has no practical applications, it advanced the understanding that quantum systems are fundamentally different from classical systems. This understanding underpins many current quantum technologies, including the quantum computer.
However, researchers have now managed to replicate this effect in a laboratory using only classical particles. To achieve this they used the strong link between particles and waves in quantum mechanics, and the fact that the function describing the superposition of a particle behaves like a wave. In their experiment, an oil droplet floating in liquid silicone represented the particle in the bomb experiment. A wave in the liquid silicone represents the wave element in the particle’s position and the tracks it left in the unchosen path, which passes through the bomb by interference.
The researchers' success rate in identifying "bombs"—represented by small blocks in the fluid’s pool—matched the expectations of the Elitzur-Vaidman experiment. Instead of using a quantum particle, which combines the qualities of a particle and a wave, the researchers used a classical particle and a regular wave in a liquid, achieving similar results.
An important accomplishment by the researchers is the use of an imitation of the quantum world to identify a bomb with classical measures - seemingly an impossible task. Mimicking nature is a valuable and important tool across all scientific fields, and is used, for example, when mimicking the structure of the human brain in machine learning algorithms or recreating the qualities of light receptors on butterfly wings to produce solar cells. Here, there is a novel application of this principle, and it is intriguing to speculate which additional quantum systems could be conceptualized using classical measures.
On the other hand, it is important to remember the significant difference between quantum and classical systems. The quantum experiment uses one component—a quantum particle that combines both particle and wave qualities. In the classical recreation, two separate components are used—one particle and one wave. Additionally, if we wish to recreate an experiment involving more quantum particles, we would need a wave composed of many components. The wave would become so complex that a system with just a few tens of particles would already be too complicated to produce.